The PNG File Format

Portable Network Graphic, or PNG, is a file format for image compression. PNG has replaced the Graphics Interchange Format (GIF) (and JPEG where documents are displayed) for use within Web pages. The GIF format is owned by Unisys, and its usage for image-handling involved patent licensing. Previously GIF users could make, view, and send files freely but they couldn’t develop software that creates them without an arrangement with Unisys. So the PNG format was developed by an Internet committee expressly to be patent-free, while also providing a number of improvements over the GIF format.

A PNG file is compressed in lossless fashion as all image information is restored when the file is decompressed during viewing. PNG files are also used to display documents and to replace the JPEG format for such uses. (JPEG works well for color photos and due to its "lossy" nature, it lets the creator make a trade-off between file size and image quality when the image is compressed. Typically, an image in a PNG file can be 10 to 30% more compressed than in a GIF format and will also look better than JPEG when displaying documents.

Advantages of PNG:

Offers a variety of image transparency options – to not only make one color transparent, but control the degree of transparency as well (also known as "opacity")

Interlacing (gradual resolution) of the image is supported and is faster in developing than in the GIF format

Gamma correction allows you to "tune" the image in terms of color brightness required by specific display manufacturers

Images can be saved using true color (24 bit color) as well as in the Web color palette and gray-scale formats provided by the GIF

Disadvantages of PNG:

Does not support animation (as GIF does), but can through a file extension called MNG

The PNG format is not as well known or accepted in the Web graphics community. On small images, GIF can achieve greater compression than PNG

PNG Image Conversion and Viewers

Snowbound's imaging products enable you to convert any format to PNG, with the option of specifying output resolution to 24 bit, 8 bit or 1 bit. Companies commonly use PNG compression for storage and Web compatibility.

The PNG format was developed in early 1995, after the realization that the algorithm used in the GIF format had been patented by Unisys. Other issues with the GIF format such as its limitation to 256 colors, made it advantageous for a replacement to be created. Although GIF allows for animation, it was decided that PNG should be a single-image format. A companion format called MNG (Multi-image Network Graphics) has been defined for animation. Developmental Timeline Highlights:

October 1, 1996: Version 1.0 of the PNG specification released

November 10, 2003: PNG is now an International Standard

March 3, 2004: recognized as a standard by the International Organization for Standardization (ISO)

October 14, ' + '1996: approved for use by the Internet Engineering Steering Group

Additional Resources:

Important:Avoid surprises and delays. Snowbound Software recommends that, as part of your evaluation process, you test your own sample documents against our products. Though we've been in the document business for almost 20 years, modifications to document formats occur from time to time. Often they don't conform to specifications, but you may find it important to work with them. Snowbound, unlike most of our competitors, develops it own technology. If in the rare case you come across a problem or non-conforming document or image, we can often quickly fix it.